1. Field of the Invention
The present invention relates to an arrangement for lubricating a countershaft brake.
2. Description of the Related Art
One type of known vehicle automatic transmissions is a two-axis (two-shaft) transmission equipped with an automatic clutch. This automatic transmission, is sometimes equipped with a brake system for applying a brake on a countershaft in a transmission housing. Further, this type of automatic transmission does not have mechanical synchronization units so that the countershaft should be decelerated by the brake for synchronization when shifting up a transmission gear.
Specifically, during the shift up operation, a speed-change sleeve is disengaged from a spline of a lower-speed speed-change gear and engaged into a spline of a higher-speed speed-change gear. In this sleeve disengagement and engagement (or synchronization), the rotation speed (rpm) of the spline of the higher-speed speed-change gear should be reduced until it matches the rotation speed (rpm) of the speed-change sleeve. To this end, the countershaft brake reduces the countershaft rotation speed as mentioned earlier.
This countershaft brake is operated every time the shift up occurs. Thus, the countershaft brake must possess anti-burning property, anti-wear property and operation reliability. For this reason, a wet type is generally employed so as to feed lubrication oil to the brake. An associated lubrication system either causes the brake to be immersed in an oil pool or feeds pressurized oil to the brake by a pump. Both of them can suppress temperature elevation of the oil so that the braking performance is maintained.
However, if the brake should be immersed in an oil pool, the lubrication system must have space for it in a transmission housing. Further, simply immersing the brake in the oil pool is not sufficient since heat generated upon braking is accumulated in the oil pool. To overcome this, the oil is required to be circulated by a certain mechanism.
If the pressurized oil should be supplied to the brake, oil passages from the oil pump to the brake must be prepared.
An object of the present invention is to overcome the above described problems. Specifically, the object of the present invention is to provide a compact and simple lubrication system for a transmission countershaft brake.
According to one aspect of the present invention, there is provided a lubrication system for lubricating a countershaft brake including a gear mounted on a countershaft of a transmission, an oil pool formed in a transmission housing sufficient to immerse at least part of the gear in the oil pool when the gear is not rotating, a plate member for obstructing oil splashed up by rotations of the gear, and an oil passage for introducing the oil obstructed by the member to the countershaft brake. Since the gear on the countershaft is utilized as a pump to feed the oil to the countershaft brake, the lubrication system does not need a separate oil pump, and therefore can have a simple structure.
The countershaft brake may include a hub fixed to an end of the countershaft and housed in a brake casing attached to the transmission housing, a plurality of friction plates provided on the hub such that they can move in an axial direction of the transmission housing, a plurality of separator plates provided on the brake casing such that they are loosely engaged with the friction plates and can move in the axial direction of the transmission housing, and a piston provided in the brake casing such that it can move in the axial direction of the transmission for pressing the friction and separator plates against each other for tight engagement of these plates.
The piston may have at least one first oil passage for introducing the oil to an interior of the piston and hub from the oil passage. The hub may have at least one second oil passage for feeding the oil to the friction and separator plates from the interior of the hub by a centrifugal force generated upon rotations of the countershaft. An inlet of the oil passage may open above a first surface of the oil pool when the gear is rotating. If the oil inlet was immersed in the oil pool, circulation of the oil would be hindered and lubrication performance would be deteriorated. An oil return passage may further be provided for returning the oil to the oil pool from the countershaft brake. A downstream end of the oil return passage may open above the first surface of the oil pool when the gear is rotating and below a second surface of the oil pool when the gear is not rotating. The oil outlet above the oil pool surface allows the oil to easily drop from the countershaft brake to the oil pool. The oil outlet below the oil pool surface allows the oil to flow back into the countershaft from the-oil-pool. Thus, even when the gear is not rotating, the countershaft brake is lubricated. The countershaft brake may be housed in a clutch housing attached to a main housing of the transmission housing in order to use free space in the clutch housing for accommodating the countershaft brake. Even if there is no sufficient room for the countershaft brake inside the main housing of the transmission, the countershaft brake can be provided without elongating a total length of the transmission.
Additional objects, benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the embodiments and the appended claims, taken in conjunction with the accompanying drawings.